The present invention relates to the surface treatment of copper foil which is used for the production of printed circuit boards, and more particularly, a process for maintaining a bright copper tone during long storage and throughout lamination under heat and pressure, while at the same time retaining the solderability and/or solder-wetability of the surface of the copper foil. A copper foil to be laminated to printed circuit boards for various electronic applications requires superior properties in order to endure subsequent processes to which the foil may be subjected in forming finished circuit boards.
A printed circuit board is ordinally formed by a process wherein a copper foil is continuously produced electrolytically, the respective surfaces of the copper foil being a mat side and a shiny side. The mat side which is faced to the electrolyte solution during electrolysis is subsequently nodularized to adhere intimately to resinous material when it is compressed at high temperature, and the shiny side is treated to prevent oxidation during storage or lamination under heat and pressure. The mat side of the copper foil impregnated with resinous material is applied to a substrate and pressed to produce a copper-clad laminate at high temperature. In many cases however, the nodularized surface of the mat side is first applied with an adhesive, dried by heating and pressed to the resinous laminate at high temperature to form a copper clad laminate.
The copper clad laminate thus produced is formed to a desired pattern by various processes such as, drilling and/or punching, pattern etching, or the like, and is soldered by dipping the same in a solder bath. Accordingly, good solderability is indispensable for the shiny surface of the copper foil for printed circuit boards. Further, the copper surface is apt to be brown-colored by oxidation by virtue of the high temperatures required to adhere the copper foil to the laminate. Even at room temperature, the copper surface is often discolored or loses gloss during storage or transportation due to oxidation. In general, consumers always require the copper clad laminate to maintain a bright copper surface.
Although various methods for treating copper foil have been developed to satisfy the above requirements, none of them can satisfy all of these requirements at the same time. By one prior art process, solderability is deteriorated and by another process, the copper surface is significantly discolored by heat. For instance, treatment with benzotriazole and electrodeposition in chromate solution are well known prior art methods for surface treatment of copper foil. However, the copper foil treated with benzotriazole is significantly brown colored by heat treatment, although it shows good solderability and storage stability. On the other hand, the electrolytic chromate treatment of copper foil is also widely used, but this has the defect of poor solderability or solder-wetability, although it can maintain a bright copper surface against heat treatment and during storage.
It is accordingly an object of the present invention to provide a process to improve the solderability of the copper surface of a copper clad laminate. Another object of the invention is to provide a process to maintain a bright copper surface of the copper laminate against heat treatment and long storage at room temperature.